This application claims priority from Korean Application No. 1998-15214, filed on Apr. 28, 1998, which claims priority from PCT/KR99/00202, filed Apr. 28, 1999, which are hereby incorporated by reference.
The present invention relates to a laser and a method for its operation, more particularly to a wavelength-swept pulse laser producing a short pulsed output with a center wavelength continuously varying with time and a method for generating such wavelength-swept laser pulses.
The present invention also relates to a wavelength-swept laser producing continuous output and a method for generating such wavelength-swept continuous laser light.
A wavelength-swept laser is a light source whose output wavelength continuously varies with time. Wide-band gain medium and wavelength sweeping means are required to operate such a wavelength-swept laser. The most efficient wavelength sweeping method is to vary the center wavelength of a wavelength tunable filter placed within a laser resonator with time.
For example, Wysocki et al. developed a laser capable of sweeping about 15 nm wavelength range at a rate of a few hundred Hz. The wavelength-swept laser used an erbium doped fiber as the gain medium and an acousto-optic filter as the wavelength sweep element(Reference: Optics Letters, Vol. 15, P879, 1990). Such a wavelength-swept laser together with a wavelength tunable laser is used in wide applications.
The wavelength-swept laser has been used in low interferometric distributed sensors, frequency range distributed sensors and fiber grating array sensors. It can also be used to analyze the wavelength characteristics of an optical device in a short time. Applications in the spectroscopy, optical communication and the like are motivated by the expectation that wavelength-swept lasers will take place of conventional wavelength tunable lasers, but examples of these applications have not been reported.
On the other hand, conventional wavelength-swept lasers have been operated with continuous wave, not with mode-locked wave. To obtain pulse type output, well-known active or passive mode-locking technology is generally required. In the case of the wavelength-swept laser, new mode-locking technology can be developed since the center wavelength of a filter placed within a laser resonator is continuously varied. However, no wavelength-swept laser has been reported that adopts such new mode-locking technology.
In general, a resonator mode of a laser is determined by the condition that the round-trip phase delay of laser light be some integral multiple of 2 xcfx80. Therefore, the frequency of m-th resonator mode has a value of fm=mc/L, where c is the velocity of light and L is the round-trip optical path of the resonator. If only single resonator mode oscillates in a laser, single frequency laser light with extremely narrow line-width is obtained. The output power of the laser light has a constant value with respect to time. If several resonator modes simultaneously oscillate, the laser output characteristics can be divided into two different states according to the amplitude and phase of the respective modes. First, if the amplitudes and phases of the modes have arbitrary relations or vary with time, continuous wave output is obtained. In this case, the power of the continuous wave output varies randomly with time due to so-called xe2x80x9cmode partition noisexe2x80x9d. Second, if both the amplitudes and phase differences of the modes are constant, short pulsed output is obtained by the coherent mode interference. This state is called xe2x80x9cmode-lockingxe2x80x9d.
There are two techniques to achieve the mode-locking. In the case of active mode-locking, modulation frequency is adjusted equal to some integral multiple of the intermode spacing of longitudinal resonator modes using amplitude modulator, phase modulator, frequency shifter or the like. In the case of passive mode-locking, a saturable absorber, its equivalent optical device or resonator configuration is used.
However, an expensive modulator or an additional saturable absorber is required for the conventional mode-locking techniques.
It is therefore an object of the present invention to provide a wavelength-swept pulsed laser capable of obtaining short laser pulses without using an expensive modulator or an additional saturable absorber and to provide a method of generating such laser pulses.
It is another object of the present invention to provide a wavelength-swept laser capable of obtaining continuous wave output and to provide a method of generating such a laser output.
In order to accomplish the aforementioned object, the present invention provides a laser, comprising: a resonator having an optical path including therein an optical gain medium capable of amplifying light over wide wavelength band, a wavelength tunable filter with minimum loss center wavelength range, and a non-linear medium with light intensity dependent refractive index; an optical pump means for the population inversion of the optical gain medium; and a filter modulation signal generating means for continuously varying the minimum loss center wavelength range of the wavelength tunable filter with time; wherein the laser output is short mode-locked pulse type and its center wavelength varies continuously with time.
In the invention, the optical gain medium can be any one selected from a rare earth ion doped single mode optical fiber, a rare earth ion doped single mode planar waveguide, a titanium doped sapphire crystal or a Ndxe2x80x94YVO4 crystal.
Moreover, a semiconductor amplifier may be used as the optical gain medium. In this case, the current supplied by a current generator pumps the semiconductor amplifier. Preferably, the current intensity is modulated at the intermode spacing of longitudinal resonator modes or some integral multiple of the intermode spacing to modulate the gain of the semiconductor amplifier, thereby helping the generation of the mode-locked optical pulses as well as adjusting the timing for the pulse generation.
In the invention, the wavelength tunable filter can be any one selected from the group consisting of an acousto-optic wavelength tunable filter, a Fabry-Perot interferometric wavelength tunable filter and a reflective refraction grating with varying reflective center wavelength depending upon rotation.
The wavelength tunable filter may include a beam deflection means and an optical device capable of producing low optical loss only within determined frequency range when the light transmitted or reflected depending on the controlled beam direction is coupled to the resonator. In this case, the beam deflection means can be an acousto-optic modulator that controls the beam direction according to the frequency of the acoustic wave. Otherwise, the beam deflection means may be a multiple phased array that controls the beam direction according to the phase differences of the respective light beams when light beams divided into several optical paths recombine together.
On the other hand, the non-linear medium may preferably include a length of single mode optical fiber or semiconductor material to enhance self-phase modulation effect and to act as a saturable absorber, thereby the non-linear medium helps the generation of mode-locked optical pulses.
However, the gain medium can act as a non-linear medium when the gain medium is a rare earth ion doped optical fiber having high non-linear coefficient or a titanium doped sapphire crystal. In this case, additional non-linear medium is not needed.
The resonator preferably includes an optical amplitude modulator for helping the generation of mode-locking as well as for adjusting the optical pulse generation timing, and a modulation signal generator for supplying alternating electrical signal to the optical amplitude modulator, the frequency of the electrical signal being equal to the intermode spacing of longitudinal resonator modes or some integral multiple of the spacing.
The optical amplitude modulator may be replaced with an optical phase modulator.
In order to accomplish the aforementioned object, the present invention provides a method of mode-locked laser pulse generation, comprising the steps of: preparing within a resonator a wavelength tunable filter and a non-linear medium with light intensity dependent refractive index; transmitting optical pulses in the non-linear medium to broaden the spectrum of the optical pulses by inducing self-phase modulation; tuning the wavelength tunable filter so that the minimum loss wavelength range of the tunable filter can continuously vary with time; and amplifying only selected portions of the broadened optical pulses, the wavelength spectrum of the selected portions being placed around the minimum loss wavelength range.
In the above method, the wavelength tunable filter is tuned so that V is higher than a critical speed Vc(=ln(r)xcex944/b2) for most of wavelength sweeping time, thereby a plurality of resonator modes can simultaneously oscillate, where V is the variation speed of the minimum loss center wavelength, xcex94 is the wavelength spacing between resonator modes, b is the full width at half maximum, ln(r) is the natural logarithm of the ratio r of the maximum to the minimum light intensity for each mode. In general, ln(r) has a value in the range of 15 to 25.
In this case, the wavelength tunable filter is preferably tuned by applying electrical signal to the tunable filter with the frequency and/or voltage of the electrical signal continuously and periodically sweeping over a predetermined range. More preferably, a short electrical pulse may be superimposed over the front portion of each repeating waveform of the electrical signal to tune the pulse generation timing to the electrical pulse as well as to help the generation of optical pulses.
In order to accomplish the aforementioned other object, the present invention provides a laser, comprising: a resonator having an optical path including therein an optical gain medium capable of amplifying light over specific wavelength band, a wavelength tunable filter with minimum loss center frequency range, and a frequency shifter shifting the frequency of the light; an optical pump means for the population inversion of the optical gain medium; a filter modulation signal generating means for continuously varying the minimum loss center frequency range of the wavelength tunable filter with time; and means for suppressing the generation of optical pulses by adjusting the frequency shift in said frequency shifter substantially equal to the center frequency change in the wavelength tunable filter for one resonator round trip time; wherein the laser output is continuous wave type and its center frequency varies continuously with time.
Preferably, the frequency shifter is an acousto-optic frequency shifter operating by an alternating electrical signal with a fixed frequency, the acoustic wave generated by the electrical signal giving the refraction and Doppler shift effect to the light.
Otherwise, the frequency shifter and the wavelength tunable filter are comprised of only one acousto-optic wavelength tunable filter. In this case, the frequency change in the electrical signal applied to the acousto-optic filter to change the center frequency of the acousto-optic filter is small with respect to time, the direction of frequency shift being the same with that of the filter center frequency change. The frequency shift for one resonator round trip may be substantially equal to the frequency change in the filter center frequency.
In order to obtain the above laser light, the present invention provides a method, comprising the steps of: preparing a resonator having an optical path including therein a wavelength tunable filter with minimum loss center frequency range, and a frequency shifter; producing a fixed amount fFS of frequency shift for the light passing through the optical path by the frequency shifter; producing continuous change fF1 in the center frequency of the wavelength tunable filter for a resonator round trip time; and tuning the wavelength tunable filter so that fFS and fF1 have a substantially same value, and the laser output spectrum within the resonator oscillates around the center frequency of the wavelength tunable filter; thereby the generation of optical pulses is suppressed and the laser radiates continuous wave.
In brief, the wavelength-swept pulse laser of the present invention is based on a new finding that sufficient amount of non-linear phase generated by the non-linear medium disposed in the resonator can induce spontaneous mode-locking.
Pulse spectrum is broadened by the self-phase modulation effect and experiences lower filter-dependent optical loss than the continuous oscillation components, thereby inducing mode-locking. According to this method, short stable optical pulses of several pico seconds are obtained without using expensive modulators or saturable absorbers.